The present invention relates to a hot-wire arc welding apparatus having a nonexpendable electrode for welding a workpiece while a filler wire heated by a voltage applied thereto is being fed to the weld, and more particularly to a control circuit for getting such a hot-wire arc welding apparatus started in a welding operation.
One generally known type of hot-wire arc welding apparatus, as shown in FIG. 1 of the accompanying drawings, comprises a tungsten electrode 1 which is supplied with a current from an arc power supply 2 for generating an arc 4 between the electrode 1 and a workpiece 3, there being a melted pool 5 formed on the workpiece 3 by the arc 4. A filler wire 6 is fed to the weld by a filler wire feeder composed of a motor 7 and drive rollers 8, through a current feeder tip 9 and an insulating guide 10. A wire heating power supply 11 applies a voltage across the current feeder tip 9, the filler wire 6, the molten pool 5, and the workpiece 3 to heat the filler wire 6 due to its resistance. When the filler wire 6 as thus heated to a high temperature is fed into the melted pool 5, the filler wire 6 can easily be melted by the heat of the arc 4 and the melted pool 5, thus forming a deposit of metal 12 on the workpiece 3. The tungsten electrode 1 extends coaxially through a shield nozzle 13. The filler wire 6 is supplied from a coil of filler wire mounted on a wire reel 14.
The voltage is applied to the filler wire 6 through a distal end (hereinafter referred to as a "feeder point") 9a of the current feeder tip 9. The filler wire 6 starts to be heated as it is fed along past the feeder point 9a and is heated to an optimum temperature when it reaches the workpiece 3. At the time of starting welding operation, a portion of the filler wire 6 which is closer than the feeder point 9a to the workpiece 3 is fed into the melted pool 5 at a temperature lower than the optimum temperature. Particularly, a tip end portion of the filler wire 6 in the vicinity of the workpiece 3 remains unheated at the time of the start of the welding operation. When such an unheated tip end portion of the filler wire 6 is fed, it plunges into the melted pool 5 or the unmelted end portion of the filler wire 6 is displaced out of the melted pool 5, with the result that welding imperfections are caused at an initial portion of the weld.
FIG. 2 illustrates a conventional control method proposed to cope with the above difficulty. According to this control method, the rate of feed of the filler wire and the current supplied for heating the filler wire are controlled to vary with time, namely, to increase gradually in synchronism with each other. The proposed method allows the filler wire to be fed at a proper rate at all times without abrupt changes in the rate as shown in FIG. 3. The abrupt changes result in a difficulty in that the filler wire, which has not been preheated to the optimum temperature, plunges into the melted pool 5. Such a conventional method, however, is disadvantageous in that it does not control the arc current, and hence the filler wire 6 becomes melted into droplet 15 prior to reaching the workpiece 3 due to the arc heat while the filler wire 6 is being fed at a low speed right after the welding operation has started. Since the filler wire 6 has a tendency to coil, the melted droplet 15 is liable to be attached to the tungsten electrode 1, and thus the device fails to produce a good weld at frequent intervals. FIG. 4 is a diagram illustrating the filler wire 6 and its tendency to coil, and FIG. 5 is a fragmentary side elevational view of an electrode, a workpiece and a filler wire, the view being utilized to illustrate a difficulty with the prior art welding apparatus.